It's been almost two years since Google added the "View as slideshow" option to Gmail, which let you view attached PowerPoint presentations right in your browser. Last week, Google updated its PowerPoint support: Now you can zoom in and out of slides, shuffle their order, select text for copying and pasting, and turn a presentation into a PDF.
Here's an example of PPT file viewing in Gmail:
Finally, Gmail's document viewer now supports TIFF images -- including multi-page TIFFs (like the ones I receive from my eFax account). That's a nice perk, as Windows' default image viewer shows only the first page.
You don't need to do anything to enable these new features -- they've already been rolled out to Gmail users. Rock on, Google!